1. Field of the Invention
The present invention relates to a multisensor system intended for controlling an implantable heart stimulator of the type having a piezoelectric pressure sensor adapted to be placed in the bloodstream of a living organism with at least one electrode adapted to be in electrical contact with said bloodstream.
2. Description of the Prior Art
In today""s pacemaker systems different physiological sensors are often used for rate control and diagnostics.
An electrochemical blood oxygen pressure sensor is described in Holmstrxc3x6m et al., xe2x80x9cTwo years follow up on a pO2-sensor controlled pacemaker: A comparative study in healthy and AV-node ablated dogsxe2x80x9d, Europace ""97, 8th European Symposium on Cardiac Pacing, Athens, Greece, 8-11 June, 1997, pp. 477-481, and Holmstrxc3x6m et al., xe2x80x9cLong term in vivo experience of an electrochemical sensor using potential step technique for measurement of mixed venous oxygen pressurexe2x80x9d, Biosensors and Bioelectronics, 13(12), December 1998, pp 1287-1295. This partial blood oxygen pressure sensorxe2x80x94pO2xe2x80x94sensorxe2x80x94includes an electrode arrangement formed by a working electrode, a reference electrode and a counter electrode, this last mentioned electrode being formed by the heart stimulator housing.
A piezoelectric pressure sensor adapted to be placed in the bloodstream of a living organism is disclosed in e.g. U.S. Pat. No. 4,600,017, and in U.S. Pat. No. 5,271,408 a system for blood flow measurements within vascular vessel s or the heart is described. This system has two transducers, one of which has piezoelectric segments mounted on the exterior surface of a catheter with outer electrodes adapted to be in electrical contact with the bloodstream.
With the aid of multisensor systems it is possible to extract more information about the body""s cardiac output need. A multisensor system for controlling a pacemaker is described in U.S. Pat. No. 5,213,098. The multisensor system according to this patent has two separated sensors, namely a ventricular or arterial blood pressure or flow sensor and a separate oxygen saturation sensor intended to be positioned in the coronary sinus. Such a multisensor system is, however, difficult to use and makes implantation complicated.
An object of the present invention is to provide a simple multisensor system including a piezoelectric pressure sensor and an oxygen pressure sensor, which avoids the drawbacks of the above-mentioned prior art multi-sensor system.
This object is achieved by a multisensor system intended for controlling an implantable heart stimulator which includes a piezoelectric pressure sensor adapted to be placed in the bloodstream of a living organism and at least one electrode adapted to be in electrical contact with the bloodstream, and wherein the electrode forms the measurement electrode of an oxygen pressure sensor.
In this manner existing electrode configurations of a heart stimulator are used for multisensor applications and a compact multisensor system of dual type is provided without changing the structure or mechanical properties of the stimulator electrode system.
In an embodiment of the sensor system according to the invention a signal processor is controlled to deliver as an output signal the pressure signal and the oxygen pressure signal from different selectable time windows of the cardiac cycle. Thus different time windows are selected for different kinds of measurements. The oxygen pressure measurement is a quick measurement and preferably is performed in a measurement window after the QRS-complex.
In the preferred embodiment the piezoelectric pressure sensor has a supporting structure provided with a layer of piezoelectric material. According to an embodiment of the sensor system of the invention the supporting structure of the pressure sensor includes one of the materials of titanium, titanium alloy, titanium nitride, platinum, platinum alloy, niobium, niobium alloy, tantalum, tantalum alloy or carbon, since all these materials are biocompatible.
These materials may for instance be used as a conducting layer between the piezoelectric layer and a non-conducting supporting structure, for improving the conductivity between the piezoelectric layer and the supporting layer and/or for improving the adhesion of the piezoelectric layer to the supporting structure. For improving the adhesion a thin layer (2-3 atom layers) of titanium or chrome-nickel is preferred.
In other embodiments of the sensor system of the invention, in which the piezoelectric element is provided with an outer electrically conducting layer which is covered by an electric insulation, a ring of the insulation is removed to form a ring-shaped measurement electrode of the oxygen pressure sensor or openings are made in said insulation to form the measurement electrode of the oxygen pressure sensor. This latter embodiment will increase the mechanical stability of the electrode.
In other embodiments of the sensor system provided for sensing the impedance between the measurement electrode and the reference electrode and supplying a corresponding impedance signal to the signal processor for selectable separation of the pressure, oxygen pressure and impedance signals, a depolarization sensor can be provided for sensing depolarization signals picked up by the measurement electrode. This depolarization sensor supplies a corresponding depolarization signal to the signal processor for selectable separation of the pressure, oxygen pressure, impedance and depolarization signals. Thus, in this way the multisensor system according to the invention is extended to include measurement of the parameters electrical impedance and depolarization without complicating or otherwise modifying the existing stimulator electrode configuration.